Lamps can use one or more artificial light sources for many purposes, including signaling, image projection, or illumination. The purpose of illumination is to improve visibility within an environment. One challenge in effective illumination is controlling the spread of light to achieve optimum visibility. For example, a single unshaded light bulb can effectively reveal with reflected light the objects in a small, uncluttered room. However, an unshaded bulb is likely to produce glare, which in turn can actually reduce visibility.
Glare occurs when relatively bright light—rather than shining onto the objects that a person wishes to view—shines directly into the viewer's eyes. Glare can result in both discomfort (e.g., squinting, an instinctive desire to look away, and/or the like) and temporary visual impairment (from constriction of the pupils and/or scattering of bright light within the eye, as examples). In most situations, glare is merely unpleasant; in some cases, it can be dangerous.
The problem of glare exists for nearly all illuminating light sources, which is why shades or diffusers are commonly used to block light from directly entering a viewer's eye. The wide range of lampshades demonstrates how common and varying the need is to block some but not all light from a light source.